1. Field of the Invention
This invention relates to roughening of p-type GaN layers and a photoelectrochemical method for etching and roughening p-type GaN layers.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Photoelectrochemical (PEC) wet etching has been applied to a variety of semiconductors, including GaAs, InP, and GaN. For GaN especially, PEC etching has been of great interest since there are very limited alternatives for room temperature wet etching.
FIG. 1 illustrates a PEC etching setup according to the present invention which includes a light source (e.g., above-bandgap 1000 Watt Xe lamp 100) and an electrochemical cell, where the semiconductor (of the e.g., GaN Light Emitting Diode (LED) sample 102) acts as the anode of the system and has metal 104 (usually platinum) patterned directly on it to act as the cathode. Light 106 generates electron-hole pairs in the semiconductor, and electrons are extracted through the cathode, while holes participate in oxidation reactions at the semiconductor surface, causing the semiconductor surface to be dissolved in an electrolyte 108. Because of the surface band bending at the semiconductor/electrolyte interface, holes are typically confined at the surface in n-type materials only, while electrons are confined at the surface in p-type materials. In addition, accumulation of photogenerated electrons at the p-type semiconductor surface constrains the etching of that material. Thus, PEC etching of p-type semiconductors has been difficult to achieve. The light 106 from the light source 100 may be filtered, for example, by a GaN filter 110, the metal 104 may be an opaque metal (e.g., Ti and Pt) mask, acting as the cathode, and the electrolyte solution 108 may be a 5 molar (M) potassium hydroxide (KOH) solution, for example. The p-GaN of the LED 102 is the anode for the PEC etching and the electrolyte 108 is in a container 112.
PEC etching is a well-established process used for n-type and unintentionally doped semiconductors, but it has had very limited success for etching p-type layers. There have been limited reports of PEC etching of p-type semiconductors in the past, but they have generally required elevated temperatures, a substantial external bias on the system, or a complex experimental apparatus. In addition, etch rates are generally very slow compared to n-type materials.
Several groups have used the application of a substantial external bias to p-type samples to achieve PEC etching of p-type semiconductors [1], [2]. Either a very large bias is necessary or the etch rate is very slow. Both of these groups used GaN. In the case of Borton et al. [1], a high-temperature KOH soak was necessary before etching, and then etching occurred under a small (˜2V) bias, resulting in etch rates on the order of 1-5 nm/minute and very rough morphology. Yang et al. [2] achieved higher etch rates (2 μm/minute) but had to apply an external bias of 10 V. Hwang et al. [3] used a chopped ultraviolet (UV) light source to achieve electrode-less etching of p-type GaN. They were able to obtain an etch rate of 2.8 nm/minute using this technique, but at the cost of increased roughness compared to similar etch conditions for n-type GaN.
Several groups have reported on the use of KOH/ethylene glycol mixtures to etch p-GaN in a purely chemical manner. However, this etch is very crystallographically selective, and also requires the use of quite elevated temperatures. The etch described in Na [5] requires a temperature of 165° C.
Deep UV irradation has been used for PEC etching of p-type GaAs [6]. This technique relies on the fact that the penetration depth of deep UV is very short in GaAs, and thus very high energy holes are generated close to the surface, are injected into the interface between the semiconductor and the electrolyte, and can participate in etching reactions there.
A two-compartment electrochemical cell has been used to PEC etch bulk p-GaAs [7]. In this case, the use of a basic solution on one side of the wafer and an acid solution on the other side of the wafer provides a driving force for holes to move towards the basic side. Light is incident on the acidic side so that electrons are confined there, while photogenerated holes diffuse to the basic side, driven by the difference in surface bandbending on the two sides, and participate in etching on the basic side in the absence of electrons. While feasible for fundamental studies of etching, this two-compartment geometry does not easily accommodate the practical etching of GaN devices.
Thus, there is a need in the art for improved processes for PEC etching p-type semiconductors. The present invention satisfies this need.